Optical fibers are presently used in a vast array of applications. As shown in FIG. 1, typical optical fibers 1 include a body 3 formed from a core region 5 formed from of higher index of refraction material surrounded by at least one cladding region or layer 7 manufactured from a material having a lower index of refraction relative to the core region 5. For example, optical fibers having two (2) or more cladding regions have been manufactured. Typically, the core region 5 and cladding region 7 of the optical fiber are manufactured from silica or glass materials. Optionally, a protective coating or material 9 may be applied to the exterior surface of the cladding 7. Light of a sufficiently low numerical aperture that is introduced into the end of the optical fiber is then guided through the fiber.
In some applications, it may be desirable to add one or more dopants to the core region, the cladding regions, or both. For example, FIG. 2 shows an embodiment of a double-clad doped optical fiber 11 having a body 13 formed from a core 15 positioned within a first cladding region or pump cladding region 17, which is located within a second cladding region 19. Optionally, a protective coating or material 21 may be applied to the exterior surface of the second cladding region 19.
In some applications, the core region 15 may be doped with one or more optically active rare-earth ions to form fiber lasers and/or fiber amplifiers. Doped double-clad fibers as described above may be extremely useful for amplifying pulses. Exemplary pulses include, without limitations, those having pulse durations ranging from nanosecond durations to femtosecond durations. In some applications, the core region 15 may be small enough and the numerical aperture (hereinafter NA) of the core region 15 low enough to permit the core region 15 to support only a single spatial mode.
It is often desirable to manufacture an optical fiber 11 having a larger core region 15 to minimize the nonlinear effects of the signal in the core region 15. Nonlinear effects can include self phase modulation, stimulated Raman scattering stimulated Brillouin scattering and four wave mixing. In these optical fibers, known as large mode area fibers (hereinafter LMA fibers), the lowest NA that can be manufactured repeatedly using conventional fiber drawing methods is about 0.060. As such, the largest single mode core region 15 is about 30 μm in diameter. The pump cladding region 17 is typically about 250 μm in diameter and has a much larger NA of 0.46. The larger diameter of the pump cladding region 17 and NA is needed to capture the pump light emitted by a pump source (not shown) in optical communication with the optical fiber 11. Often, laser diodes, which tend to output highly divergent, multimode pump beams, are used as pump sources.
Typically, to minimize nonlinear effects, a short length (about 1 or a few meters) of doped double-clad optical fiber is used to form a fiber amplifier. However, it is also desirable to absorb all or at least most of the pump signal to increase efficiency. The effective absorption of the pump signal is determined by the doping level in the core region 15 and the ratio of the size of the pump cladding region 17 to the core. Since the pump cladding region is undoped, the pump signal is typically only absorbed when the pump signal encounters the core region 15 and thus the effective value of the absorption is decreased by the cladding to core area ratio.
Typically, for these LMA fibers, the maximum doping level is often limited due to photodarkening, which leads to power degradation over time in highly doped (and thus highly excited) fibers. In combination with the typical cladding to core ratio of 8:1, a typical fiber length to absorb the pump is approximately 2 meters.
Recently, some fiber amplifier manufacturers have developed doped double-clad rod-type fiber devices in an effort to minimize the nonlinear effects associated with conventional doped double-clad fibers. For example, NKT Photonics has developed a rod-type doped double-clad fiber offering reduced nonlinearity. Different manufacturing processes are employed to manufacture these rod-type devices. As a result, the NA of these rod-type device can be as low as 0.02 and single mode cores of 85 μm or 100 μm can be manufactured. In addition, by using a web of air holes formed around the pump cladding, rod-type fibers having a cladding NA of 0.6 are presently available. Unfortunately, a number of shortcomings of rod-type fiber device have been identified. For example, the cladding to core ratio of rod-type fiber architectures is limited by the need to use small air holes for guiding the light in the core. For example, an 85 μm core may be manufactured with a 200 μm pump cladding. In another example, a 100 μm core may include a 285 μm cladding. As such, the smallest cladding to core ratio rod-type fiber devices is about 2.35. As a result, the typical length of the rod-type device needed to absorb the pump light is 0.5 to 1 meter. Due to the low NA, rod-type doped double-clad fiber devices typically cannot be bent without causing significant bending losses to the signal traversing through the fiber device.
In light of the foregoing, there is an ongoing need for a double-clad optical fiber having a large core with a low NA that supports only a single mode or, in the alternative, that supports only a few modes. Furthermore, such a fiber that can efficiently absorb the pump beam in a short length is desired.